1. Field of the Invention
The invention relates to the particleboard art generally with particular reference to a product that is comprised of a core layer of wood particle-adhesive aggregate hot pressed between two thin face layers of similar composition to form a lightweight laminate-like structural component.
2. Description of the Prior Art
In conventional industrial and commercial building construction in the eastern United States, the roof structure of a typical building consists of 18 to 22 gauge wide-ribbed steel deck fastened to supporting steel beams which are spaced 4 to 6 feet on center. Applied to the top surface of the ribbed steel deck are boards of rigid insulation which form the base for a standard built-up roof. Until now, no particleboard of sufficient bending stiffness and low cost has been manufactured that could compete with ribbed steel decking in this market.
At a time when there is increasing concern over the disappearance of nonrenewable resources such as iron ore and fossil fuels, and when the volume of hardwoods is increasing, the use of a hardwood structural particleboard instead of wide-ribbed steel decking as a roofing panel is highly desirable. A product of this type must meet the structural requirements of the construction industry. It would utilize small and inferior trees that are indigenous to the Eastern United States, the targeted marketing area, and have limited use in other products. The invention should require less energy for manufacture and have better insulating properties than steel decking. Transportation costs would be cut because of the wood's proximity to the targeted marketing areas.
A similar invention is U.S. Pat. No. 3,164,511, issued Jan. 5, 1965, Oriented Strand Board, hereinafter referred to as Elmendorf. Elmendorf claimed a structural particleboard comprised of three or more laminae, the face laminae being comprised of "parallel oriented" or alined flakes, and the core lamina being comprised of "randomly distributed" flakes. However, Elmendorf's invention anticipated only particleboard with densities greater than those of the wood species of which they are comprised and particleboard component woods with specific gravities of less than 0.6. Elmendorf, column 3, lines 23-27.
Maloney stated that "[t]he most important species variable governing board properties is the density of the wood raw material itself . . . " In general terms, the lower density woods will produce panels within the present desired specific gravity ranges, usually with strength properties superior to the higher density species. In the United States and Canada, this specific gravity range of presently used species is from about 0.30 to 0.50. Some of the higher density species such as oak and hickory, which are about 0.65 specific gravity, are used in higher density products, usually of the hardboard classification. Maloney, T. M. Modern Particleboard and Dry Process Fiberboard Manufacturing. San Francisco, Miller Freeman Publications, 1977. p. 161. TS875.M29. Kelly, referring to the effect of compaction ratio on physical properties, asserts: "In general, a conventional particleboard with a density lower than the density of a wood furnish will be unsatisfactory." Kelly, M. W., Critical Literature Review of Relationships Between Processing Parameters and Physical Properties of Particleboard. Madison, Wis., Forest Products Laboratory, USDA, Forest Service, 1977. p. 3. 1.9.F761G.FPL 10. High density hardwoods have not been used as a major constituent in the production of medium density particleboard.
Canadian Pat. No. 584,444, issued Oct. 6, 1959, Particle Board, hereinafter referred to as Yan, described a method of assuring uniform density in the layers of a particleboard. It did not, however, show that a medium density board having sufficient bending stiffness to permit use in structural applications could be manufactured.
U.S. Pat. No. 3,478,861, issued Nov. 18, 1969, Orienting Wood Strands, hereinafter referred to as Elmendorf II, described a method of alining flakes in the face layers of particleboard such that the lengths of the flakes are parallel with the length of the board. However, the Elmendorf II process will not produce medium-density boards from dense hardwoods with bending stiffness approaching that of our particleboards.
A prospectus published by the Department of Forestry and Natural Resources of Purdue University and authored by two of the three inventors and others in April 1977, did not disclose the invention, but merely indicated that a market existed for such an invention and suggested that a particleboard constructed in a certain manner could conceivably meet the construction industry requirements.
The inventors are unaware of any other structural particleboard with the physical properties required for the intended long-span roof decking application that is cost-competitive with wide-ribbed steel decking; nor are they aware of any high-strength structural particleboard manufactured at a particleboard density less than or equal to the density of the wood species.